In a heavy saucepan whisk together the water and the cornstarch and simmer the mixture over moderate heat, whisking, for 2 minutes. Add the milks, the salt, and the sugar, heat the mixture over moderately low heat, whisking, for 1 to 2 minutes, or until the sugar is dissolved, and remove the pan from the heat. Let the mixture cool completely, stir in the Guinness, and freeze the mixture in an ice-cream freezer according to the manufacturer's instructions.